A. Field of the Invention
This invention relates to an apparatus and method for measuring the concentration of gas present at the inlet membrane of the apparatus and more particularly to a dualchamber sensor and pump for accurately measuring the concentration of gas by means of ionizing the gas.
B. Description of Prior Art
It is known to measure the concentration of a gas by means of using an ion pump to create a vacuum in a chamber, ionizing the gas and measuring the current within the ion pump produced by the ionization of the gas as a reflection of the concentration of the gas. This method normally utilizes a thin metal membrane or diaphram (interface) immersed in a solution (either a liquid or gas mixture) which permits the passage of gas through the membrane and into the vacuum chamber. The gas ionized in the pump is attracted to a cathode which is made of gettering material, is sorped thereunto, and is converted into an electrical current. This method is described, for example, in U.S. Pat. No. 3,683,272, issued Aug. 8, 1972 to Vissers et al. entitled "Method and Apparatus for Determining Hydrogen Concentration in Liquid Sodium Utilizing Ion Pump to Ionize the Hydrogen". This technique, however, has several well-known disadvantages, which are discussed, for example, in Vissers et al., "Hydrogen-Meter Leak Detector for LMFBR Steam Generators", published by Argonne National Laboratory in October, 1973. See also Turner, et al. "Penning Discharge Getter-Ion Pumps", I.E.E.E. Trans. on Nuclear Science, Volume NS-14, No. 3 (June 1967). These disadvantages relate to the fact that the pumping speed of the ion pump changes during the course of the measurement and this in turn affects the results of the measurement itself. A case where the equipment influences the subject of the test is a highly undesirable situation. More specifically, in the case of pumping hydrogen, unwanted hydrides tend to form on the cathode. The rate at which these hydrides form and hydrogen is sorbed and goes into solution with the cathode material is dependent on the surface condition of the cathode material. Chemical compounds formed by other gases and the cathode material and other forms of contamination may drastically alter the ability of the cathode to pump hydrogen. Pumping sites, that is, locations on the cathode where reactions take place, change. Attempts have been made to precondition the cathodes by argon sputtering to stabilize the cathodes but these approaches have met with only limited success. Another problem is that the diffusion rates in the bulk metal of the cathodes are variable. Consequently the pumping speed within the pump is a function of the history of the pump, the time it has been used, and how it has been used. All of this means that the ion pump current changes with time, and may in turn indicate a change in pumping speed of the pump, masking changes in the concentration of the gas at the instrument interface.
Also, with the exception of unwanted ion current changes stemming from changes in pump speed, the sensitivity of known instruments is fixed. If one changes the size of the pump by either increasing or decreasing the number of discharge cells or elements, the total ion current will be invariant for the same concentration of gas in solution or chemical reaction rate at the instrument interface. What is needed is a more sensitive and accurate device to measure the concentration of the gas at the gas-instrument interface in which the result is independent of the equipment being used.